As a private landlord you will be required to give up your property empty when you sell it back to the Council. This is called "Vacant Possession". It is therefore very important that you properly terminate the tenancy agreement with your tenant and that you keep them informed of the timescale of selling your flat back to the Council, so that your tenant can make alternative arrangements for their housing.
If you are using a standard Assured Shorthold Tenancy Agreement, you must give your tenant 2 months notice. You may wish to take advice from your Solicitor as to how to do this.
In some circumstances, for example where your tenant has children living as part of the household, the Council may have responsibilities for their rehousing. This will need to be assessed by the Housing Choices Team at Perceval House. It may be useful for them to discuss their circumstances with the Council Regeneration Team in the first instance.
Further informtion can also be found using - https://www.gov.uk/leasehold-property/leaseholder-rights-and-responsibilities